An ultrasonic diagnostic apparatus is such that transmits an ultrasonic signal from its ultrasonic probe into an examined body, receives the signal echoing from tissues or a diseased part of the examined body, and displays a tomographic image formed from the echo signal on a CRT for diagnostic use.
In such an ultrasonic diagnostic apparatus, in the case where the frame rate is low, i.e., where the interval is large between the periods in which the ultrasonic wave is transmitted, if the change between the signal received one frame before and the signal currently received is great, there is produced a flicker on the CRT screen. In order to improve the situation, a method called an inter-frame averaging process to average the previous and current data has been in use.
In the inter-frame averaging process which has so far been in practice, arithmetic averaging of the previous frame data and the current frame data has been performed using a constant arithmetic-averaging coefficient at all times. Accordingly, there have been such problems that, when the examined object is a moving reflector such as a valve of the heart and a flow of blood or when the ultrasonic wave is transmitted and received while the ultrasonic probe is being moved, the characteristic of the apparatus to follow up an image of an internal organ or the like is deteriorated and an afterimage is produced. If the weighting with the arithmetic-averaging coefficient .omega. is lightened to improve the situation, a difficulty arises that noises increase and a flickering view is produced.
On the other hand, there has been a method in which the absolute value of the differential between the old data and the new data is taken and the value of .omega. is varied according to the absolute value and such a variable value of .omega. is used in an arithmetic averaging process. In this method, while the arithmetic-averaging coefficient .omega. is varied for each frame according to a detected movement of the echoing part, the averaging process for the entire object becomes lighter when there is a moving part in the image and, hence, the averaging process for the portion where there is no movement also becomes lighter and such a difficulty arises that noises cannot be sufficiently removed.